Optical intensity modulators are important devices for optical communication, i.e., for imparting information onto laser light. They typically have the ability to modulate the intensity of an incident narrow-line (continuous-wave) laser signal from close to 100% of its nominal maximum value to close to 0%. In wavelength-division-multiplexed (WDM) systems, a modulator is typically needed for each wavelength, so it is desirable to have wavelength-selective modulators that can be cascaded, such that each is able to modulate a select wavelength without affecting other wavelengths. Preferably, such modulators can be integrated on a chip in large numbers, to scale with large numbers of wavelengths used. Therefore, modulators should also be small and energy-efficient. Energy-efficiency is particularly important for on-chip optical interconnects for multi-core microprocessors, and for on-chip optical transmitters on microprocessors as part of either all-on-chip or part-on-chip/part-off-chip optical networks between processors and/or between processors and memory. Finally, there are applications where a modulator with two output ports is employed to provide the modulated output signal (e.g., 1000101) in one port, and the complementary signal (i.e., 0111010) in the other port.
In general, modulators are designed to: (i) maximize modulation speed; (ii) minimize the energy required to modulate; (iii) minimize the optical bandwidth occupied by the modulation in order to allow cascading of modulators without crosstalk and with minimal wavelength spacing; and (iv) minimize the driving signal (to avoid material breakdown or to be able to use typical voltage levels available with CMOS driving circuits, on the order of 1V to 5V), which simultaneously aids in minimizing the energy required to modulate. In general, there is a trade-off between achieving these goals—in particular (i) and (ii), usually referred to as the sensitivity-bandwidth trade-off. Achieving high-speed modulation calls for photon lifetimes shorter than the modulation period, and is associated with a broad modulation bandwidth requiring strong actuation signals (e.g., voltages or currents). On the other hand, high sensitivity of the modulation to the driving signal typically requires sharp amplitude changes within a narrow optical bandwidth range of the device. Then, weak spectral shifting in the spectral response caused by weak modulation may be sufficient to substantially shift the sharp spectral feature across the fixed-wavelength input wave. Consequently, there is a limit to simultaneously achieving high speed and high sensitivity.
Energy-efficient modulators may be optically resonant structures, such as silicon microring resonators coupled to a waveguide, or Mach-Zehnder interferometers assisted by a ring resonator in at least one of the interferometer arms. Such modulators have a number of drawbacks. Structures using a cavity coupled to a waveguide have a Lorentzian response, which means that even when they are loss-less, i.e., have 100% transmission on resonance to the drop port, they do not roll off to a full zero transmission off resonance. As a result, they typically require that the resonance be moved during modulation by more than one bandwidth in order to achieve practically low transmission (that gives large on-to-off contrast, i.e., extinction ratio), which is energetically costly. Further, the extinction ratio of these devices may deteriorate when loss is associated with the frequency shift during modulation. This is the case, for example, when modulation is achieved with carrier injection in silicon, i.e., using the carrier-plasma effect. Modulators using a ring-loaded Mach-Zehnder configuration have the drawback of using multiple 3 dB splitters, which typically cause substantial losses on resonance. This is because a 3 dB splitter is difficult to design and realize to be lossless, and, furthermore, symmetric splitters approaching 50%:50% splitting (3 dB) have higher losses in general than asymmetric splitters with weak splitting approaching 0%:100%. Namely, the loss is typically considerably smaller than the smaller of the two output fractions. Furthermore, such modulator devices may have 3 dB loss off resonance at all wavelengths and may, therefore, be unsuitable for direct cascading in a WDM system.
Accordingly, a need exists for an improved resonant optical modulator.